Conventional dense wavelength division multiplexing (DWDM) systems employ a laser in combination with a series of modulators, which may serve the function of manipulating the amplitude, phase, and/or frequency of the laser to create an optical signal based on a received electrical signal. Multiple modulators may be employed to multiplex several different electrical signals, such as radio frequency (RF) signals, onto a single optical signal by employing each modulator or modulator pair, depending on implementation, to alter a specific wavelength of light. The resulting optical signal may then be placed on an optical fiber for transmission.
One approach to creating a transmitter for use in a DWDM system is to use discrete modulators. However, this implementation method may need a complex and expensive transmitter architecture encompassing a large number of optical functions and elements. This complexity problem has led to research into large-scale photonic integration, which would allow the integration of multiple optical functions and wavelength channels onto a single device or block (sometimes also referred to as a circuit component). The integrated approach may reduce architecture complexity, increase system reliability, and reduce system power consumption compared to systems employing discrete optical components.
A Mach-Zehnder modulator (MZM) based on a Mach-Zehnder interferometer may be a key component for high-speed optical transmitters and is frequently used in DWDM networks. The MZM may be made up with waveguides in a good optical material, whose refractive index may be altered by applying an electric field on one or two arms of the interferometer. Intensity and/or phase modulation may be obtained by creating phase differences between two arms. Depending on the overall phase difference, the output light recombines more or less efficiently, or does not recombine at all, at the output of the interferometer, conducting to a modulation of the output power. The MZM may be combined with a continuous wavelength (CW) laser in a transmitter to allow precise manipulation of optical signals in both amplitude and phase. As the advent of coherent optical communications, MZM may become more important as it is a reliable modulating device capable of supporting coherent technologies.
Traditional MZMs may typically be made of lithium niobate (e.g., LiNbO3). However, lithium niobate is highly anisotropic, which requires its input ports to be positioned in the opposite direction from its output ports. Further, the anisotropic nature of a traditional MZM may prevent significant on-chip waveguide bending. The design constraints have forced all previous attempts at large scale photonic integration to rely on a monolithic architecture. The monolithic structure may require routing of incoming electrical signals related to the MZMs at the center of the monolith, which may become very complex given a relatively high number of MZMs integrated. This in turn creates an upper limit on the number of channels available to a monolithic system. In high-speed systems, the difference in length between the simple short routing to the outer MZMs when compared to the longer complex routings to the inner MZMs may also cause a difference in the propagation of the associated electrical signals. The difference in propagation delay needs to be considered by other components of the system, creating further system complexity and potential increase in cost.